The present invention relates to a presensitized printing plate for waterless lithographic printing, in particular offset printing, and also to a process for the production thereof. The printing plate according to the invention has a substrate, a silicone rubber coating which repels printing ink and an outer, radiation-sensitive coating.
Presensitized printing plates for waterless lithographic printing are known, it being possible for the ink-repellent coating to be situated above or below the photosensitive coating. If the photosensitive coating is disposed as the uppermost coating, the plate has the advantage that it can be developed substantially more easily than in the converse case; it is substantially more difficult to develop a plate with an ink-repellent covering coating, for example of silicone rubber, by removing the areas of the photosensitive coating soluble in the developer and the areas of the silicone coating situated above them. Usually the addition of fairly large quantities of organic solvent to the developer or a developer based on organic solvents and generally mechanical assistance by rubbing, brushing or the like are necessary. The developer is contaminated by the insoluble silicone rubber particles and therefore has to be renewed more frequently.
Despite these known disadvantages, plates with a silicone coating as uppermost coating have hitherto mainly prevailed in practice. This is due, inter alia, to the difficulty of anchoring a photosensitive coating on a cured silicone rubber coating so it adheres firmly. In the prior art, in addition to many printing plates with this coating arrangement, a number of printing plates have also been described for waterless offset printing in which the photosensitive coating is the outer coating.
DE Pat. No.-B-1,571,890 describes a dry lithographic printing plate in which a toner image or a photosensitive coating, which is processed by exposure to light and development to yield the printing ink-receptive image, is applied to a cured silicone elastomer coating. Optionally, a prefabricated metal coating is pressed onto the silicone elastomer coating and then provided with a photosensitive coating. An oleophilic image is produced on the oleophobic silicone coating by exposure to light, development and etching. Without the intermediate coating, only low print runs are obtained, while with the relatively thick copper interlayer, the run is high, but a high resolution, as with the usual offset plates, cannot be obtained.
DE Pat. Nos.-A-2,039,901 and 2,361,815 describe a process for producing a waterless printing offset plate in which a photosensitive coating and an uncured silicone elastomer coating are produced on separate coating bases, then laminated and heated to cure the silicone coating. The process is laborious and requires the use of very narrowly limited materials.
U.S. Pat. No. 3,632,375 describes waterless printing lithographic printing plates which carry an anchoring coating on a silicone rubber coating and a photosensitive coating thereon. The anchoring coating is mostly composed of water-soluble polymers which are optionally crosslinked in addition. The anchoring coatings and the photosensitive coatings are mostly hydrophilic and therefore accept ink badly. The developer resistance of these materials is also low.
U.S. Pat. No. 3,728,123 describes a similar material in which the intermediate coating is itself light-hardenable.
DE Pat. No.-B-2,449,172 describes a presensitized lithographic printing plate for waterless lithographic printing which carries a photo-polymerizable coating on a silicone elastomer coating. A similar material is described in U.S. Pat. No. 4,225,663.
In the known materials for producing waterless printing lithographic printing plates which carry a photosensitive coating on the ink-repellent silicone coating, the adhesion between the coatings is always problematical. If the silicone coating is cured or vulcanized before the application of the photosensitive coating, the wetting of the latter coating becomes difficult and the adhesion is particularly bad. If it is cured after application, there is a risk of such good adhesion of the anchoring of the oleophilic coating parts that the coating parts are not completely removed during development and the silicone coating accepts ink. If an anchoring coating is provided between the two coatings, it is so similar in its nature and composition to the usual constituents, for example, binders, in the photosensitive coating that similar problems arise. In particular, according to the prior art, it is not possible, or only possible in a very laborious manner, to anchor very thin, high-resolution, photosensitive coatings with such good adhesion that highly resolved, well-adhering image stencils, from which high runs can be printed, are obtained by simple development.
German Patent Application P 3,628,719.9 (DE Pat. No.-A-3,628,719 not prepublished) describes a presensitized printing plate for waterless lithographic printing which has a photosensitive coating, an intermediate coating of amorphous silicic acid and a covering coating of silicone rubber on a coating base.